1. Field of the Invention
The present invention generally relates to a method of treatment of plants. More particularly, the invention relates to an improved method of plant pest control.
2. Background of the Prior Art
Pesticides are chemicals designed to combat the attacks of various pests on agricultural and horticultural crops. They fall into three major classes: insecticides, fungicides and herbicides (or weed killers). There are also rodenticides (for control of vertebrate pests), nematicides (to kill microscopic eelworms), molluscicides (to kill slugs and snails) and acaricides (to kill mites).
Pesticides may also be divided into two main types, namely contact or nonsystemic pesticides and systemic pesticides. Contact or surface pesticides do not appreciably penetrate plant tissues and are consequently not transported or translocated, within the plant vascular system. The earlier insecticides, fungicides and herbicides were of this type; their disadvantages are that they are susceptible to the effects of weathering (wind, rain and sunlight) over long periods and new plant growth will be left unprotected and hence open to attack by insect and fungal pests. The early agricultural fungicides were, therefore, protectant fungicides--in other words they are designed to prevent the development of the fungal spores, but once the fungus has become established and infection starts to ramify through the plant tissues such nonsystemic fungicides possess little eradicant action and usually cannot halt the infection.
In contrast, many of the more recent pesticides are systemic in character--these can effectively penetrate the plant cuticle and move through the plant vascular system. Examples are provided by the phenoxyacetic acid selective herbicides, certain organophosphorus insecticides and the more recently discovered systemic fungicides like benomyl.
Systemic fungicides are also sometimes termed plant chemotherapeutants and can not only protect the plant from fungal attack, but also cure or inhibit an established infection. They are little affected by weathering and will also confer immunity on all new plant growth.
Pests can be divided into various groups. In the plant kingdom, characterized by the ability of the organism to photosynthesize carbohydrates from air and water with the aid of the green pigment chlorophyll, higher plants growing where man does not want them are termed weeds and are important pests. Of the lower plants, algae are not generally of as great importance as pests, although in some circumstances, e.g., in lakes and other slow moving water, excessive algal growth or "bloom" may cause considerable damage and require treatment with chemicals (algicides).
Fungi or nonphotosynthetic plants cannot obtain their nutrients from air and water since they do not have chlorophyll; consequently they feed directly on decaying plant or animal matter (saprophytic fungi) or on living plants or animals (parasitic fungi). There are thousands of different species of fungi mainly found in soil--some, like yeasts, are unicellular while others are composed of a network of branched filaments (hyphae). A number of fungi are serious pests attacking both living crop plants and also crops in storage.
Several bacteria are causal agents of plant diseases, although they are not nearly as important as the phytopathogenic fungi. Bacteria can be observed under the microscope and can be classified according to their shape; thus a spherical bacterium is termed a coccus while a rod-shaped one is a bacillus.
Viruses, like bacteria and fungi, attack plants and animals and some species cause significant plant diseases. Viruses form a distinct category of living organism because they are not true cells. Unlike bacteria they are too small (100-300 A) in diameter to be observed with an ordinary microscope, but they can be revealed under the electron microscope--each virus consists of a single strand of DNA or RNA surrounded by a protective coat of protein.
Several higher animals (vertebrates) are important pests, e.g., mice, rats and rabbits; another group of pests is represented by the true insects (arthropods) which are invertebrates. The latter possess three pairs of legs and the adult body has three parts; the arachnids (mites and ticks) differ from true insects in having no distinct division of the body into three parts; also they usually have four pairs of legs. In the lower orders of animals, certain nematodes, parasitic worms often with unsegmented bodies, are important crop pests.
If pesticides are to be active they must reach the ultimate site of action within the target organism. Thus even surface fungicides, like Bordeaux mixture, must be able to penetrate the fungal spore; similarly contact insecticides have to penetrate the insect cuticle, and contact herbicides the plant cuticle when they impinge on it. The requirements if the pesticides are to be systemic in action are much more stringent because in addition they must have the capacity to be absorbed by the roots or leaves or seeds of plants and be delivered to other parts of the plant. In this way the whole plant, including new growth, is protected from fungal attack, or rendered poisonous to any insect that eats or sucks it.